This is in Bard's Tale I: Tales of the Unknown.
Harkyn's Castle is in the northwest of Skara Brae, located at N24 E4. You will have to defeat a minimum of 2 guardian statues to get here: a Golem and a Grey Dragon. Grey Dragons are serious foes who can deal 32-128 damage to your entire party with a breath weapon, so unless you can quickly deal around 50 damage, it will annihilate you.
You're supposed to do this dungeon after the Cellar, Sewers and Catacombs, but you can come here as soon as you can kill the dragon to get the achievement if you're in a hurry.